"どうして　まだ　晩御飯を食べませんか？" is correct. "どうして　まだ　晩御飯を　食べますか？" It's a bit hard to understand what you are trying to say..x( but it's close!

「どうして〜しないのですか」expression. どうして（まだ）晩御飯を食べないのですか？ 　　　　　　　　　　　　食べないの？ are natural. But we can understand if you say, どうしてまだ晩御飯を食べませんか？ But I feel a little strange. The latter one is completely wrong.

both stands One sentence is "why do not est dinner yet" Why you eat dinner more (not enought why ) mean is different both stand but remember " まだ“　means not only " yet" but also "More." You meant YET made a setence yet 2nd is not correct.